The present invention relates to an X-ray reflector which is used in a lens reduction projection aligner for the manufacture of semiconductor devices and, more particularly, to an X-ray multilayer film reflector which exhibits high reflectivity for soft X-rays of wavelengths ranging from several to tens of nanometers.
Conventional soft X-ray multilayer film reflectors usually have a multilayer structure formed by depositing two kinds of materials of different refractive indexes to predetermined thicknesses alternately through use of a vacuum evaporation, sputtering or similar method.
Conventionally, the two materials are chosen mainly by the following three criteria so as to obtain a soft X-ray multilayer film reflector of great reflectivity. First the two kinds of materials do not much absorb X rays; second, they have greatly different refractive indexes; third, they have a definite boundary without any reaction or diffusion between them and the interface remains smooth.
Since the refractive index of each material depends on the wavelength of soft X rays as well, combinations of materials which fulfill the above-noted criteria are limited for each specified wavelength. Some materials, when combined, are liable to react with each other at the mutual interface to form an intermediate product and some materials are also apt to incur interdiffusion. In the case where the multilayer structure is formed using a combination of materials which are liable to interaction or interdiffusion, the interface gets rough or the boundary gets indefinite owing to the interaction or interdiffusion between the materials used. In such an instance, the intended reflectivity cannot be obtained, or the interaction gradually proceeds and the multilayer structure is destroyed, resulting in the performance being impaired. Such combinations of materials do not fulfill the aforementioned third criterion and cannot be employed because of the above-noted defects, even if they are expected to provide a high reflectivity.